U.S. patent number 4,701,050 [Application Number 06/702,329] was granted by the patent office on 1987-10-20 for semiconductor exposure apparatus and alignment method therefor.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Naoto Nakashima, Toshihiko Nakata, Yoshitada Oshida, Masataka Shiba, Sachio Uto.
United States Patent |
4,701,050 |
Oshida , et al. |
October 20, 1987 |
Semiconductor exposure apparatus and alignment method therefor
Abstract
A semiconductor focusing exposure apparatus in which an opposite
face of a mask to a face to be illuminated by exposure light is
illuminated with alignment light so that the light reflected from
said opposite face may be used for alignment and which is equipped
with a second moving arrangement which is separate from a moving
arrangement for an x-y moving table supporting a wafer, for
aligning the mask and the wafer in an orthogonal direction with
respect to the optical axis of a focusing lens. Moreover, the
center of the flux of alignment pattern light for illuminating the
wafer is made incident upon a line of intersection on which a plane
containing the optical axis of an alignment optical system and the
optical axis of said focusing lens and the incident plane of said
focusing lens intersect with each other. Still moreover, the
optical path of the alignment light beam is aligned in parallel
with a straight line joining an alignment mark formed on the
diffraction pattern and the center of the entrance pupil of said
focusing lens.
Inventors: |
Oshida; Yoshitada (Fugisawa,
JP), Shiba; Masataka (Yokohama, JP),
Nakashima; Naoto (Yokohama, JP), Nakata;
Toshihiko (Yokohama, JP), Uto; Sachio (Yokohama,
JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
27322704 |
Appl.
No.: |
06/702,329 |
Filed: |
August 5, 1985 |
Foreign Application Priority Data
|
|
|
|
|
Aug 10, 1984 [JP] |
|
|
59-166597 |
Aug 31, 1984 [JP] |
|
|
59-180531 |
Sep 21, 1984 [JP] |
|
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59-196645 |
|
Current U.S.
Class: |
356/139.07;
250/202; 250/548; 250/557; 356/152.2 |
Current CPC
Class: |
G03F
9/7049 (20130101) |
Current International
Class: |
G03F
9/00 (20060101); G01B 011/26 (); G01C 001/00 () |
Field of
Search: |
;356/152,141
;250/202,201 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Buczinski; Stephen C.
Assistant Examiner: Koltak; Melissa L.
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
What is claimed is:
1. A semiconductor exposure apparatus comprising:
a mask;
an exposure light source for illuminating a pattern on said
mask;
a focusing lens for focusing the pattern of said mask on a
wafer;
alignment means having at least a lens, a mirror and array sensor
for detecting the relative positions of an alignment mark formed on
said mask and an alignment mark formed on said wafer;
a first means for moving an x-y table supporting said wafer;
a second means for moving said alignment means in an orthogonal
direction with respect to the optical axis of said focusing lens;
and
means for introducing an alignment light, through the face of said
mask, opposed to a face illuminated by exposure light.
2. A semiconductor exposure apparatus according to claim 1, wherein
the alignment light is produced by a reflection of a mirror pattern
formed on said mask.
3. A semiconductor exposure apparatus according to claim 1, wherein
the alignment light is produced by a reflection on a mirror pattern
which is formed on said mask and which has a diffraction
pattern.
4. A semiconductor exposure apparatus according to claim 1, wherein
the alignment light is produced by a reflection on a mirror pattern
which is formed on said mask and which has a hyperbolic
pattern.
5. A semiconductor exposure apparatus according to claim 1, wherein
said second means for moving said alignment means in an orthogonal
direction with respect to the optical axis of said focusing lens
and a mechanism for moving the same in a tangential direction with
respect to said orthogonal direction are integrally
constructed.
6. A semiconductor exposure apparatus comprising:
a mask;
an exposure light source for illuminating a pattern on said
mask;
a focusing lens for focusing the pattern of said mask on a
wafer;
alignment means for detecting the relative positions of an
alignment mark formed on said mask and an alignment mark formed on
said wafer;
a first means for moving an x-y table supporting said wafer;
a second means for moving said alignment means in an orthogonal
direction with respect to the optical axis of said focusing
lens;
means for introducing an alignment light, through the face of said
mask, opposed to a face illuminated by exposure light; and
means for making the center of the flux of the alignment light for
illuminating the alignment pattern of said wafer incident upon a
line of intersection between a plane containing the optical axis of
an alignment optical system and the optical axis of said focusing
lens and the entrance pupil plane of said focusing lens.
7. A semiconductor exposure apparatus according to claim 6, wherein
said incidence means is made movable in association with said
orthogonal moving means.
8. A semiconductor exposure apparatus comprising:
a mask;
an exposure light source for illuminating a pattern on said
mask;
a focusing lens for focusing the pattern of said mask on a
wafer;
alignment means for detecting the relative positions of an
alignment mark formed on said mask and an alignment mark formed on
said wafer;
a first means for moving an x-y table supporting said wafer;
a second means for moving said alignment means in an orthogonal
direction with respect to the optical axis of said focusing
lens;
means for introducing an alignment light, through the face of said
mask, opposite a face illuminated by exposure light;
a diffraction pattern formed as an alignment mark on said mask;
and
means for aligning the optical path of the light beam, which comes
from an alignment optical system for illuminating said alignment
mark formed of the diffraction pattern, into parallel with a
straight line joining said alignment mark and the center of the
entrance pupil of said focusing lens.
9. A semiconductor exposure apparatus according to claim 8, wherein
said diffraction pattern is formed of a hyperbolic pattern.
10. A semiconductor exposure apparatus according to claim 8,
wherein said diffraction pattern is formed of a Fresnel zone
pattern.
11. An alignment method in a semiconductor exposure apparatus, of
aligning an alignment mask formed on a wafer with an alignment mark
formed on a mask by focusing a pattern of said mask on said wafer
by means of a focusing lens, comprising the steps of:
moving an alignment optical system for said alignment in orthogonal
directions with respect to the optical axis of said focusing lens
and in tangential directions with respect to said orthogonal
directions; and
aligning the optical path of an alignment light beam, which is
emitted from an alignment optical system for illuminating the
alignment mark formed of a diffraction pattern on said mask, into
parallel with a straight line joining said alignment mark formed of
the diffraction pattern and the entrance pupil of said focusing
lens.
12. A semiconductor exposure apparatus according to claim 8,
wherein said alignment means includes:
an alignment light source fixed relative to said focusing lens;
a concave lens;
a convex lens fixed on a stage, which carries an alignment optical
system for said alignment thereon for moving the same in an
orthogonal direction with respect to the optical axis of said
focusing lens, and having both its distance (M) from said mask and
its focal length (N) made equal to the distance (L) between the
incident eye of said focusing lens and said mask;
means for moving the focal point of said convex lens on a straight
line extending in an orthogonal direction of said focusing lens
through the focal point of said concave lens at the side of said
focusing lens; and
means for aligning the optical path of a light beam, which comes
from an alignment optical system for illuminating said diffraction
pattern, into parallel with a straight line joining said alignment
mark formed of the diffraction pattern and the center of the
entrance pupil of said focusing lens.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor exposure apparatus
for exposing a wafer to the pattern of a mask and, more
particularly, to a semiconductor exposure apparatus which is
enabled to achieve an exposure, while being freed from complicated
processes for changes in the chip size of said wafer or for
advancement of the exposure process, always in the exposure
position, i.e., without any movement of said wafer after the wafer
and the mask have been aligned.
When a semiconductor is to be exposed to a pattern, according to
the prior art, the alignment between the mask and the wafer is
conducted either by a method such as the off-axis alignment method,
using an alignment optical system other than an exposure focusing
lens or by a method such as the TTL alignment method, using an
exposure focusing lens. The former method is performed by measuring
several positions of the peripheral chip on a wafer by means of an
alignment optical system and a laser length meter, by computing the
chip exposure position on the wafer on the premise that the other
chips are accurately arrayed and that the relative positions on the
optical axes of the exposure focusing lens and the alignment
optical system are accurately known, by moving the chips on the
wafer to the computed positions, and by exposing them successively
by a stp and repeat method. This method may suffice to measure the
several positions of the chips such that the time period required
for the chip alignment occupies a short duration in the time period
required for exposing all of the wafer but it fails to accurately
aligning each chip with the mask. This may raise a serious problem
in the future when the exposure pattern becomes finer and finer so
that a more accurate alignment is required. The latter method of
the prior art for conducting the alignment for each chip by means
of the exposure focusing lens is divided into several methods. One
example of these methods is shown in FIG. 1. In the example of FIG.
1, a mask 1 has its exposure circuit pattern 4 and alignment marks
112 and 112' are displaced on its face so that an alignment optical
system and an exposure optical system do not interfere with each
other when the alignment and the exposure are to be executed. For
this purpose, as shown in FIG. 2, the alignment between the mask
and the chip of the wafer is conducted by aligning the chip
alignment marks 122' (and 122) of the wafer and the alignment
images 112' (and 112) twice in the order of FIGS. 2(a) and (b), by
reading out the wafer position in x and y directions by means of
laser length meters LM (and LM'), and by exposing a chip 21 to the
image 4' of the circuit pattern of the mask, as shown in FIG.
2(c)-, on the basis of the data read out. In FIG. 1, reference
numerals 160, 161 and 161' indicate members for detecting the
position of said wafer. As compares with the off-axis alignment
method, the TTL alignment method of the prior art herein after
called a first alignment moving means has a superior alignment
accuracy but has to shift the position of the wafer for execution
of the alignment and for the exposure so that the error in the
measurement of the laser length meter leads to reduction in the
alignment accuracy. There arises another problem in that the time
period required for exposing the whole wafer is elongated.
The prior art is exemplified by Japanese Patent Laid-Open Nos.
52-109875, 55-108743, 57-142612 or 58-112330, Japanese Application
Nos. 58-243866 and 58-219415 and U.S. Pat. Application Ser. No.
684,292.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor
exposure apparatus to solve the aforementioned problems of the
prior art by making use of an exposure focusing lens, to expose the
whole wafer for a short time period instantly in the position where
the alignment has been conducted, and to ensure the alignment in
the exposure position even for changes in the size of the circuit
pattern.
In order to achieve the above-specified object, according to the
present invention, a semiconductor exposure apparatus comprising: a
focusing lens for focusing a pattern of a mask on a wafer; an
illuminating system for exposing the wafer to the mask pattern and
illuminating the wafer; and alignment means, equipped with a money
mechanism for detecting directly or indirectly the relative
positions of an alignment mark on the mask and an alignment mark on
the wafer herein after called a second alignment moving means,
which is composed of an optical system and a detecting element for
detecting the image of the wafer or the images of the wafer and the
mask and which are in a fixed positional relationship, in a
tangential direction with respect to the optical axis of said
focusing lens. The moving mechanism is used for the second
alignment moving means as a whole when the chip size is changed or
when the superposed exposure process advances so that the alignment
is to be conducted by making use of a new wafer alignment mark.
When a circuit having a different chip size is to be exposed, the
alignment means is brought as a whole toward or apart from the
optical axis of the focusing lens so that the alignment mark
disposed in the vicinity of the circuit periphery may be aligned
(in the y direction when the alignment marks 112 and 122 are used
in FIG. 1). When one circuit is to be fabricated, the mark patterns
are successively repeated for repeated superposed exposures,
whereupon the position of the new alignment mark is generally moved
in the x direction, as viewed in FIG. 1 (when the alignment marks
112 and 122 are used). After this movement of the alignment means
has been conducted, the alignment means is not moved but fixed, and
the alignment is executed in the state of the relative positions of
the mask and the wafer when the wafer is exposed to the mask. A
method of conducting the alignment in that exposure position has
already been applied for patent by the present inventors, as is
disclosed in Japanese Patent Applications Nos. 58-243866 and
58-219415. The inventions of the above-specified Applications have
already been applied for U.S.A. patent (now bearing U.S. patent
application Ser. No. 684,292) on Dec. 20, 1984.
Another object of the present invention is to solve the problem of
the displacement of the image of an alignment mark on a wafer,
which is caused by chromatic aberration when light having a
wavelength different from the light for the exposure is used as the
light having the wavelength necessary for the alignment in the
aforementioned apparatus for the alignment in the exposure
position. For this solution, the present invention is characterized
by forming the alignment mark on the mask of a later-described
diffraction pattern so that the optical path of the light beam
coming from an alignment optical system for illuminating said
alignment mark formed of the diffraction pattern may be aligned
parallel with a straight line joining the diffraction pattern and
the center of the entrance pupil of the focusing lens.
Still another object of the present invention is to correct the
misalignment which is caused when the detected position of the
wafer fluctuates in a vertical (or z) direction with respect to the
wafer face, as will be described hereinafter, in the aforementioned
apparatus for the alignment in the exposure position. For this
solution, the present invention is characterized by making the
center of the flux of the alignment light for illuminating the
alignment pattern of said wafer incident upon a line of
intersection on which a plane containing the optical axis of the
alignment optical system and the optical axis of the focusing lens
and the entrance pupil plane of said focusing lens intersect with
each other.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing the construction of the
semiconductor exposure apparatus of the prior art;
FIG. 2 is a view showing the alignment state in the apparatus shown
in FIG. 1;
FIG. 3 is a perspective view showing one embodiment of the
semiconductor exposure apparatus of the present invention;
FIG. 4 is a top plan view showing the mask in the apparatus shown
in FIG. 3;
FIG. 5 is a top plan view showing the exposure region on a wafer in
the apparatus shown in FIG. 3;
FIG. 6 is a perspective view showing an exposure apparatus of the
present invention;
FIGS. 7 and 8 are top plan views showing the relationship between a
mask and an alignment optical system in the apparatus shown in FIG.
6;
FIG. 9 is a top plan view showing the relationship between a
pattern on the mask and a pattern on a wafer;
FIG. 10 is an explanatory view showing the position in which the
image of the alignment mark of the wafer is formed due to the
chromatic aberration of a hyperbolic pattern portion;
FIG. 11 is a perspective view showing the alignment optical
system;
FIG. 12 is a perspective view showing the positional relationship
between the mask and the alignment mark;
FIG. 13 is a diagram showing the sectional optical intensity
distribution of a diffracted image by the hyperbolic pattern
portion;
FIG. 14 is a side elevation taken in the direction of arrow R from
FIG. 11 when the movement of the alignment optical system in the
tangential direction is zero (i.e., when the chromatic aberration
of the focusing lens is zero);
FIG. 15 is a side elevation taken in the direction of the arrow R
from FIG. 11 when chromatic aberration of the focusing lens
occurs;
FIG. 16 is a diagram of the alignment optical system for the mask
shown in FIG. 11;
FIG. 17 is a perspective view showing the construction of the
alignment optical system;
FIG. 18 is a diagram showing the optical path of the alignment
optical system;
FIGS. 19 and 22 are front elevations showing the alignment optical
system;
FIG. 20 is a diagram showing the alignment optical path viewed in a
front elevation;
FIGS. 21 and 23 are enlarged diagrams showing the vicinity of a
point P.sub.3 ;
FIG. 24 is a top plan view showing the alignment optical system
according to one embodiment of the present invention;
FIG. 25 is a perspective view showing the alignment optical system
using a Fresnel zone alignment, and
FIG. 26 is a view showing a Fresnel zone alignment mark.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of the present invention will be described in the
following with reference to FIGS. 3 to 5. A mask 1 has its circuit
pattern 4 focused in a reduced scale onto a wafer 3 lying on a
wafer X-Y table 70, by the action of a focusing lens 2. For this
exposure, illuminating light 14 is emitted by an exposure
illuminating system L so that light 42 having transmitted through
the circuit pattern on the mask and having been diffracted is
incident upon an entrance pupil 35 of the focusing lens. In order
that the circuit forming its image on the wafer may be focused in
high resolution on the wafer, the construction has to be made such
that an alignment optical system may not come into the outermost
periphery of the envelope which is formed by (a number of) lines
joining the periphery of the entrance pupil 35 of the focusing lens
and the periphery 11 of the circuit pattern portion. In the prior
art, therefore, the alignment mark 112 of the mask is disposed
apart from the circuit portion 4, as shown in FIG. 1, so that the
alignment cannot be made in the exposure position. In the present
embodiment, the alignment mark of the mask is formed, as shown in
FIGS. 3 and 4, with the later-described mask alignment of pattern
shape shown in FIG. 8 adjacent to the periphery 11 of the circuit
portion 4. This target is formed at its central portion with a
mirror surface 31 and at its two sides with hyperbolic patterns or
quasi-hyperbolic patterns 34 as shown in FIG. 14. Moreover, the
corresponding position of the alignment mark on the wafer provides
the so-called "scribe area" at the boundary between the chips shown
in FIG. 3. The wafer is already recorded thereon in the scribe area
with an alignment mark 7 having a width of about 6 microns and
having its longitudinal direction directed generally toward the
circuit center. The detection of the wafer pattern is conducted by
means of the laser beam which is emitted from alignment detecting
means 8 and incident upon the aforementioned mirror surface 31 on
the mask so that its reflected beam illuminates the wafer alignment
mark 7 through the focusing lens. This laser beam 19 is guided from
the outside of the alignment detecting means and is reflected by a
beam splitter B2 and a mirror 20 and then it is swung by a galvano
mirror 21. This swing method defined herein is means for improving
the S/N ratio of the alignment detection. The swing method is
disclosed in detail in the above-specified Japanese Patent
Application No. 58-243866 and U.S. patent application Ser. No.
684,292, which were already filed by the present Inventors, and its
detail is omitted here because it is not especially necessary here.
The laser beam swung by the galvano mirror 21 transmits through a
beam splitter B1 and a mirror 30 and is reflected by the
aforementioned mirror surface 5 on the mask to illuminate the wafer
alignment mark while having its angle of incidence changed. The
laser beam reflected by the wafer reverses its initial optical path
to transmit through the beam splitter B1 until it is detected by a
solid array sensor 32. If the laser beam traces that path, the
alignment detecting means can be arranged without shielding the
exposure light for printing the circuit pattern so that the wafer
alignment mark can be detected in the exposure position.
FIGS. 6 and 7 are views schematically showing the major portion of
FIG. 3 and the example structure which is equipped with two sets of
stages for moving the alignment optical system. The summary of the
moving stages will be described in the following so as to further
the understanding of the structure of FIG. 3.
First of all, as shown in FIG. 6, when the circuit pattern 4 on the
mask 1 is guided onto the wafer 3 through the focusing lens 2 by
the step and repeat method so as to effect the exposure chip by
chip (for each exposure region), it is necessary to align the
pattern 4 of the mask and a pattern 6 on the wafer 3. For this
necessity, the alignment is conducted with the alignment mark 5 on
the mask and the alignment mark 7 on the wafer by means of
alignment optical systems 8 and 8'. Reference numeral 14 indicated
exposure light. The aforementioned alignment optical system 8 and,
8' are carried on stages 9 and 9' for moving the alignment optical
system 8 and 8' in a orthogonal (T) direction and on stages 10 and
10' for moving the same in a radial (R) direction which means a
tangential direction with respect to said orthogonal direction.
FIG. 7 shows the case in which the optical axis planes of the
alignment optical systems 8 and 8' are positioned in normal planes
13 and 13' joining the optical axis 12 of the focusing lens.
In the aforementioned embodiment shown in FIGS. 3 to 5, the chips
on the wafer are exposed in a superposed manner successively to
different patterns by replacing the masks. If, in this case, an
identical alignment mark on the wafer is used for each exposure,
the pattern is degraded so that the alignment accuracy is reduced.
As the superposed exposure process advances, therefore, it becomes
necessary to use a new wafer alignment mark. This new wafer
alignment mark is exemplified by wafer alignment marks 7A and 7A'
shown in FIG. 5. These marks are positioned adjacent, at a distance
.DELTA..sub.1, to the alignment marks 7 and 7' which have been used
for the previous exposures. The symbol .DELTA..sub.1 indicates 100
to 200 microns, for example, on the wafer. In order to detect those
new alignment marks, in the present invention, the alignment
detecting means is moved in its entirety such a distance (e.g.,
M.DELTA..sub.1 if the magnification of the lens is indicated at M)
on the mask as corresponds to that distance .DELTA..sub.1. This
movement is conducted by using the alignment detecting means moving
mechanism 9. The alignment mark of the mask to be used when a new
circuit pattern is exposed by using the new wafer alignment mark 7
is indicated at 5A in FIGS. 3 and 4. Since the alignment detecting
means is moved the distance M.DELTA..sub.1 in the x direction,
i.e., in the orthogonal direction with respect to the optical axis
of the focusing lens 2, it is possible to align the wafer and the
mask absolutely like the aforementioned alignment by making use of
the mask alignment mark 5A which is displaced the distance
M.DELTA..sub.1 from 5.
Next, the following description is directed to the case in which
the external size (i.e., the chip size) of the circuit pattern is
changed in the present embodiment. If the external shape of the
chip is changed from 6 to 6L, as shown in FIGS. 3 and 5, the
external shape of the circuit portion of the mask is also changed
from 11 to 11L. In accordance with this change, the mask alignment
mark is also moved from 5 to 5L. In order to conduct the alignment
in the exposure position by making use of that target mark 5L, in
the embodiment of the present invention, the whole alignment
detecting means is moved, as shown in FIG. 3, a distance
.DELTA..sub.2 in a y direction, i.e., in the radial direction with
respect to the optical axis of the focusing lens 2 by the moving
mechanism thereof. Then, the alignment of the alignment marks 7 and
5 can be made in the exposure position absolutely like the
aforementioned method.
In a highly integrated semiconductor resist, there are used a
resist containing a material for absorbing light of the same
wavelength as that of the exposure light and a multi-layered
resist. In order to ensure the alignment in the same position as
the exposure position, in the present invention, the light used for
the alignment has a wavelength different from that of the
aforementioned exposure light. If the alignment optical systems 8
and 8' are moved for the alignment a distance X, as shown in FIG.
8, by making use of the light having a wavelength different from
that of the exposure light, the image P.sub.31 of the alignment
mark 7 on the aforementioned wafer 3 is displaced by the
aforementioned influences of the chromatic aberration of the
focusing lens from the image position P.sub.30, as shown in FIG.
10, so that it leaves the mask. As a result, if the alignment
optical systems 8 and 8' are disposed below the mask 1, as shown in
FIG. 6, the image P.sub.3 formed by the reflection on the face of
the mask 1 is to be aligned. Here, if the aforementioned alignment
optical systems 8 and 8' are arranged such that the distance taken
in the tangential direction of the aforementioned focusing lens 2
is designated at X, as shown in FIG. 10, such that the distance
from the mask 1 to the entrance pupil 35 disposed in the focusing
lens 2 is designated at L, and such that the chromatic aberration
of the focusing lens 2 is designated at .DELTA.L, the alignment
marks 7 and 7' on the wafer 3 are subjected to such a displacement
.DELTA.X by the chromatic aberration of the focusing lens 2 as is
expressed by the following equation. ##EQU1##
In order to effect the alignment while neglecting the
aforementioned displacement .DELTA.X of the alignment mark 7 on the
wafer 3, the present invention is characterized by forming a
diffraction pattern on the mask 1 in place of the aforementioned
alignment marks 5 and 5' of the mask 1, forming the diffracted
image of the diffraction pattern in the position of the image
P.sub.3 formed by the reflection on the mask 1, and by moving the
alignment optical systems 8 and 8' to align the focal point of the
light with the position of that image P.sub.3 so that the images of
the alignment marks 7 and 7' on the wafer 3 may be aligned with the
alignment marks 5 and 5' of the mask 1.
The present invention will be described in the following in
connection with the embodiments thereof with reference to the
accompanying drawings. In order to reflect the aforementioned
images of the alignment marks 7 and 7' on the wafer 3, the
alignment marks 5 and 5' on the mask 1 are formed, as shown in FIG.
12, a mirror portion 131 and a hyperbolic pattern portion 134
adjacent to that mirror portion 131. The latter hyperbolic pattern
portion 134 is formed of a group of hyperbolic or quasihyperbolic
lines.
FIG. 11 shows the construction of the alignment optical system 8.
First of all, if a light beam 132' in the direction of arrow 129
illuminates the aforementioned mirror portion 131 shown in FIG. 12
through the beam splitter B1, an enlarging lens 63, a relay lens 64
and the mirror 30, the images of the alignment marks 7 and 7' on
the wafer 3, which are formed in the position indicated at P.sub.3
in FIG. 10, are detected by the sensor 32 through the
aforementioned mirror 30, relay lens 64, enlarging lens 63 and beam
splitter B1. If a light beam 132 indicated by arrow 130 comes in
parallel with the aforementioned light beam 132' to illuminate the
hyperbolic pattern portion 134 (See FIG. 12) on the mask 1 through
a concave lens 128, a convex lens 127 and a mirror 36, it is
diffracted by that hyperbolic pattern portion 134 to form a linear
diffracted image 133 in a position indicated at P.sub.3 in FIG. 12.
The optical intensity section 133' of the diffracted image at this
time has such a distribution as is shown in FIG. 13. Thus, when the
distance X shown in FIG. 10 is zero, the images of the alignment
marks 7 and 7' of the wafer 3 are reflected by the mirror portion
131 on the mask 1 and focused at the position P.sub.3, as shown in
FIG. 14. The images of the alignment marks 7 and 7' of the wafer 3
formed at that position P.sub.3 are formed on that center line 138
of a light flux 137, which extends on the center line of the
entrance pupil 35 formed in the focusing lens 2. As a result, if
the aforementioned illumination light 132 illuminated on the
hyperbolic pattern portion 134 on the mask 1 and coming from the
alignment optical systems 8 and 8' is directed in parallel with the
aforementioned center line 138 of the light flux 137, the image
133, which is formed as a result that the light beam 132 from the
alignment optical systems 8 and 8' is diffracted by the hyperbolic
pattern portion 134, can be superposed generally on the images of
the alignment marks 7 and 7' of the wafer 3, which are formed in
the position P.sub.3. In case the aforementioned distance X is
large, as shown in FIG. 15, on the other hand, the aforementioned
images of the alignment marks 7 and 7' of the wafer 3 are formed in
the position P.sub.3 which is displaced by .DELTA.X of the
extension when the center line 138 of the light flux 137 is
reflected on the mirror portion 131. If, therefore, the light beam
132 illuminating the hyperbolic pattern portion 134 of the mask 1
and coming from the alignment optical systems 8 and 8' is directed
in parallel with the center line 138 of the light flux 137 like the
foregoing case, the image 133, which is formed as a result that the
aforementioned light beam 132 of the alignment optical systems 8
and 8' is diffracted, as indicated at 136, by the hyperbolic
pattern portion 134, can be superposed upon the alignment marks 7
and 7' of the wafer 3, which are formed in the position P.sub.3, so
that it can be detected in the same field of view as that of the
alignment optical systems 8 and 8'. Thus, the direction of the
aforementioned light beam 132 from the alignment optical systems 8
and 8' in parallel with the center line 138 of the light flux 137
is effected, as shown in FIGS. 11, 12 and 16, by first making the
light beam, which comes from the alignment optical systems 8 and 8'
to illuminate the hyperbolic pattern portion 134 on the mask 1 in
the direction 132 and by then fixing it with respect to the
focusing lens 2 so that the light beam 132 may take a relationship
to transmit substantially through the center of the focusing lens
2. Moreover, a concave lens shown in FIG. 16 is fixed on the
radially moving stage 10 so that the aforementioned light beam 132
may become independent of the aforementioned orthogonal movement of
the alignment optical systems 8 and 8' with respect to the focusing
lens 2. The aforementioned light beam 132 forms a virtual
condensation P at a point P which is spaced a focal length from the
concave lens 128. If one convex lens 127 is placed on the
orthohgonal moving stages 9 and 9', it is displaced the distance X
from the illuminating light 132 accordingly as the alignment
optical systems 8 and 8' are displaced the distance X in the
orthogonal direction with respect to the focusing lens 2. Here, if
the focal length of the aforementioned convex lens 127 is
designated at N and if the convex lens 127 is positioned at a
spacing of the distance N from the aforementioned point P, the
light beam emanating from the convex lens 127 transmits in parallel
with a straight line 140 joining the center of said convex lens 127
and the point P. Since the images of the alignment marks 7 and 7'
of the wafer 3 are located on the straight line joining the center
point P.sub.30 (which should be referred to FIG. 10) of the mirror
131 on the mask 1 and the center of the incident eye 35 of the
focusing lens 2, the inclination of that light with respect to the
mask 1 is expressed by X/L if the distance between the mask and the
incident eye 35 of the focusing lens 2 is designated by L. In order
to make the aforementioned light beam 132 have the inclination X/L,
N=L has to hold because X/N=X/L. In order to align the center of
the light beam 132 with the aforementioned center point P.sub.30 of
the mirror portion 131 on the mask 1, moreover, M=L has to hold if
the distance between the convex lens 127 and the mask 1 is
designated as M. Thus, the aforementioned diffracted image 133
formed by the hyperbolic pattern portion 134 on the mask 1 can be
aligned generally with the position P.sub.3 in which the images of
the alignment marks 7 and 7' of the wafer 3 are to be formed.
Incidentally, it is conceivable that a small displacement between
the images of the alignment marks 7 and 7' of the wafer and the
image 133 diffracted at 136 by the hyperbolic pattern portion 134
of the mask 1 may be caused by the errors of the lenses 64 and 63,
the convex lens 127 and the convex lens 128 arranged in the
aforementioned alignment optical systems 8 and 8'. This
displacement can be corrected in advance as the offset intrinsic to
the alignment by means of software. On the other hand, the method
of forming the aforementioned images of the alignment marks 7 and
7' of the wafer 3 by the diffraction can be exemplified by using a
two-dimensional Fresnel zone alignment or the like.
As has been described hereinbefore, according to the present
invention, there can be attained the result that the alignment
optical systems can be moved either in the orthogonal direction or
in the radial direction with respect to the focusing lens, and that
the alignment between the mask of the semiconductor exposure
apparatus and the pattern of the wafer can be made very accurate,
even when the aforementioned alignment optical systems are
moved.
Next, another embodiment of the present invention will be described
in the following in connection with the aforementioned focusing
displacement due to the chromatic aberration. It is desired that a
resist applied to the wafer be insensitive to the laser beam used
for detecting the alignment of the wafer alignment pattern. To
achieve this, the laser beam used has a wavelength of about 500 nm
or longer. As a result, the focusing lens generates chromatic
aberration (in the direction of the optical axis) when exposed to
that light.
FIGS. 17 and 18 show the construction of the alignment optical
system and one example of the fundamental optical path thereof.
The laser beam 19 is used for illuminating the wafer. The beam
having passed through the mirror 20 is swung by a galvano mirror
121. The beam having been converged to a position F.sub.1 by a lens
22 is converged again to a position F.sub.2 in a lens 28 through a
lens 23, mirrors 24 and 25, a polarizing beam splitter 26 and a
.lambda./4 plate 27. Moreover, the beam passes through a lens 29,
the mirror 30 and the mask 1 until it is converged to a plane
F.sub.4 of the incident eye 35 of the focusing lens 2. As a result,
a parallel beam illuminates a point P.sub.4 (or the alignment mark
7) on the wafer 3. The conjugate points of the position P.sub.0 of
the galvano mirror 121 are P.sub.1, P.sub.2, P.sub.3 and P.sub.0 so
that the plane F.sub.4 is swung (by the aforementioned swing
method), as indicated at 33, whereas the position P.sub.4 is swung,
as indicated at 34.
The image of the alignment mark of the wafer 3 is then focused on
the sensor 32 through the mask 1, the mirror 30, the lenses 29 and
28, the .lambda./4 plate 27 and the polarizing beam splitter 26 by
way of the mirror 31. The position of the sensor 32 corresponds to
the conjugate point of the point P.sub.1 of FIG. 18. In order to
enhance the optical efficiency, incidentally, the incident light is
subjected to a P-polarization to transmit straight through the
polarizing beam splitter 26, whereas the return light is subjected
to an S-polarization and is reflected.
In case a light having a wavelength different from that of the
exposure light is used for the alignment, the position of the point
P.sub.3 is spaced a distance l from the mask 1 (as shown in FIG.
18), because the chromatic aberration of the focusing lens 2
occurs.
Now, in the optical system of FIG. 18, the wafer illuminating light
is swung in the radial direction of the focusing lens 2 but is left
as a thin beam in the orthogonal direction. FIG. 19 is a front
elevation showing the alignment optical system, i.e., the sectional
state taken in the orthogonal direction from the focusing lens. A
light flux 38 emanating from the alignment optical system 8 through
the lens 29 and the mirror 30 illuminates the wafer at the position
P.sub.4 through the focusing lens 2. Since, however, the flux 38 of
the alignment light beam fails to pass not only through a line 15
of intersection, on which the plane 13 containing the optical axis
of the entrance pupil of the focusing lens 2 and the optical axis
of the alignment optical system, i.e., the plane extending downward
of the Drawing sheet in the front view intersects the entrance
pupil plane 35, but also through a line directed downward of the
Drawing sheet in the front view, it is inclined with respect to the
wafer so that the center of the flux of the return light passes
through a path 39 and is superposed at the point P.sub.3 on the
center of the flux 38 of the illuminating light to form the wafer
image at the intersecting point until it is incident at an
inclination upon the alignment optical system 8.
FIG. 20 is an expansion of a portion of the alignment optical
system 8 of FIG. 19. The system from the sensor 32 to the conjugate
plane 40 of the alignment optical system is fixed. As a result,
when the wafer 3 fluctuates in a Z direction, the image P.sub.3 of
the wafer does not always come to the conjugate plane 40 of the
sensor.
FIG. 21 enlarges the behavior in the vicinity of the point P.sub.3
when the wafer fluctuates in the Z direction. The point P.sub.3 is
in the conjugate plane 40, when the wafer is positioned, as
designed, but comes to a position P'3 with a displacement .DELTA.Z'
when the wafer fluctuates in the Z direction. At this timer, the
sensor detects a spot position P.sub.3 " on its conjugate plane 40
so that an alignment error .DELTA.T is caused.
In order to eliminate this alignment error, it is necessary to hold
the spot across over the conjugate plane 40 of the sensor always in
a fixed position irrespective of the movement of the wafer in the Z
direction.
Generally speaking, the emanating eye of the focusing lens of the
projecting exposure apparatus is in an infinitely remove position.
If, more specifically, the wafer illuminating light 38 passes
through the straight line 15 on which the plane containing the
center of the entrance pupil 35 of the focusing lens 2 intersects
with the entrance pupil plane 35, as shown in FIG. 22, the angle of
incidence upon the wafer becomes normal (as at P.sub.4) in front
view of the alignment optical system, and the return path is the
reversed optical path.
FIG. 23 shows the vicinity of the spot P.sub.3 in an enlarged
scale. Since the illuminating light and the center of the flux of
the reflected light from the wafer pass through the same path as
the optical path 38, the position in the conjugate plane 40 of the
sensor is constant even with a shift in the image plane due to the
wafer fluctuations .DELTA.Z'. As a result, no alignment error is
caused.
The embodiment of the present invention will be described more
specifically. FIG. 24 is a top plan view showing the portion
relating to the wafer illumination of the alignment optical system.
When the optical axis plane 51 of the alignment optical system is
displaced in parallel with the tangential (i.e., T) direction from
the optical axis plane 50 of the mask 1 and the focusing lens, the
conjugate point of the center F.sub.4 of the entrance pupil is
formed at the position F'.sub.3 and further at the position
F'.sub.2. If, therefore, the mirrors 24 and 25 are placed on a
small stage 54 in the alignment optical system 8 so that they are
moved simultaneously in the T direction with a stroke proportional
to the movement of the alignment optical system, the light from an
optical axis 53, which intrinsically passes through the optical
axis plane 51 so that it is condensed at the position F.sub.2,
passes through an optical axis plane 52 and is condensed at the
position F'.sub.2 until it passes through the position F.sub.4,
i.e., the center of the entrance pupil. If the galvano meter 21 is
driven, the position F.sub.4 moves in the plane 13 containing the
entrance pupil.
Since the movement of the stage 54 carrying the mirrors 24 and 25
is proportional to that of the alignment optical system 8 in the
tangential direction, this tangential movement may be reduced by
means of a wedge or lever mechanism and transmitted to the stage
54.
Similar effects can also be realized by moving the lens 23 finely
in the tangential direction. In order that a method using the
Fresnel zone pattern may be used as the alignment method to effect
the alignment of the exposure position, as shown in FIG. 26, the
center 61 of the flux of the wafer illuminating light has to be
aligned with the center P of the entrance pupil of the focusing
lens 2 in case an alignment optical system 60 is moved in the
orthogonal or radial direction, as shown in FIG. 25. reference
numeral 62 indicates a laser; numerals 63, 64 and 65 lenses;
numeral 66 a half mirror; numerals 67 and 68 mirrors; numeral 69 a
sensor; and numeral 70 a Fresnel zone pattern. Alignment can be
realized if the lens 63 is moved in the orthogonal (T) direction in
proportion to the movement of the alignment optical system 60 in
the same orthogonal direction.
Although the mask alignment mark shown in FIG. 3 is used in the
aforementioned embodiment, the present invention is not limited to
that configuration. For example, when alignment is conducted with
the ray of a g line or a ray near the same, there is adopted a
method of detecting the alignment mark itself on the mask as the
image. In this case, it is apparent that alignment can be realized
in the exposure position by moving the whole alignment detecting
system of the present invention. In this embodiment, only the
mirror surface is used as the mask pattern of FIG. 3; it has no
hyperbolic pattern at its two sides, but its peripheral position is
used as the mask pattern.
Although the mask mirror surface is used as a portion of the
alignment detecting optical system in the present invention, the
present invention can also be applied to the case in which the
detection is made by providing a separate mirror surface.
Furthermore, the present invention detects the wafer and the mask
by means of the common optical system but can be applied to the
case in which they are detected by separate optical systems. In
this modification, for example, the mask pattern is always arranged
in the fixed position on the mask so that it may be detected by the
fixed mask pattern detector, or the mask pattern is moved in
parallel in accordance with the displacement of the wafer alignment
pattern so that the mask pattern detector may be moved in parallel
with the wafer pattern detector.
As has been described hereinbefore, according to the present
invention, the alignment for each chip required for the exposure of
the fine circuit pattern having a high mounting density can be
realized in the exposure position. As a result, the alignment
accuracy achieved reaches as high as 0.1 microns, and the exposure
can be made instantly after the alignment so that the output
achieved can be about two times as high as that of the prior
art
Another important feature of the present invention is that the
alignment in the exposure position can be realized without being
troubled by the changes in the chip size, the focal displacement
due to chromatic aberration or fluctuations of the wafer in the Z
direction.
* * * * *